The present invention relates to an electric motor control device that controls power supplied from an inverter to an electric motor and regenerative power supplied in the opposite direction, and in particular to control of an operating voltage supplied from an inverter to an electric motor and a voltage control mode. The electric motor control device according to the present invention may be used, for example, in an electric vehicle (EV) that drives wheels with the electric motor, and in a hybrid electric vehicle (HEV) that includes, in addition to the electric motor, a fuel engine and a generator (occasionally referred to as electric motor or dynamotor) rotationally driven by the engine.
As the rotation speed of an electric motor increases, a counter electromotive force generated in a stator coil increases. This makes it increasingly difficult to supply a target current from an inverter to the stator coil, and hinders obtaining a target torque output. In this case, it is possible to output the target torque at a higher rotation speed by reducing a d-axis current id and a q-axis current iq which have been calculated to achieve the given motor target torque, although the efficiency of power use reduces. Such control is referred to as field weakening control.
Japanese Patent Application Publication No. JP-A-H10-66383 discloses a motor drive control device that does not utilize field weakening control to avoid a power loss and a reduction in the system efficiency due to field weakening control. The motor drive control device includes a voltage increasing circuit that increases the operating voltage applied to an inverter. Power is supplied from the voltage increasing circuit to the inverter in the case where the battery voltage is short for a target operation of a motor. Japanese Patent No. JP-B-3746334 discloses a motor drive control device including a circuit that increases the battery voltage. The motor drive control device calculates an increased voltage required for the target operation and the speed electromotive force of a motor to control the voltage increasing circuit so as to achieve the calculated increased voltage. Japanese Patent Application Publication No. JP-A-2005-210772 discloses a motor drive control device that controls an inverter and a voltage increasing circuit so as to achieve such a field weakening current and a voltage increase ratio that minimize the sum of a field weakening loss and a voltage increase loss in a region in which the target rotation speed exceeds a base rotation speed Rb. Japanese Patent Application Publication No. JP-A-2003-33071 discloses a motor control device that detects each of the power loss of a converter which serves as a voltage increasing circuit and the power loss of an inverter. The motor control device controls the converter and the inverter such that the detected power losses balance each other only in a field weakening control region, and controls the current such that the motor drive voltage is in a predetermined range outside the field weakening control region.
In an electric vehicle (EV) or a hybrid electric vehicle (HEV) having a voltage increasing function according to the related art, for example, the DC link voltage (a voltage after being increased by a converter: the secondary voltage of the converter) applied to an inverter which drives an electric motor is determined, in accordance with the target torque and the rotation speed of an electric motor, to be three-phase PWM (SVpwm) or two-phase PWM (Dpwm) in a region in which a field weakening current is not required, and to be 0 (with the modulation ratio Mi being constant at 0.707 and with the voltage control mode being Dpwm) in a region in which a field weakening current is required, in order to reduce a power loss due to the field weakening current. For example, in the case of a constant torque, the secondary-side target voltage Vuc* is determined such that the DC link voltage increases at a constant rate to reach an upper limit Vmax along with an increase in the rotation speed was shown in FIG. 6. After the DC link voltage reaches the upper limit Vmax, it is conceivable to perform field weakening control, and then to switch the voltage control mode from Dpwm to a one-pulse mode in which rectangular-wave power is applied to all the phases.